stedolan-jq/docs/content/3.manual/manual.yml

headline: jq Manual
body: |

  A jq program is a "filter": it takes an input, and produces an
  output. There are a lot of builtin filters for extracting a
  particular field of an object, or converting a number to a string,
  or various other standard tasks.

  Filters can be combined in various ways - you can pipe the output of
  one filter into another filter, or collect the output of a filter
  into an array.
  
  Some filters produce multiple results, for instance there's one that
  produces all the elements of its input array. Piping that filter
  into a second runs the second filter for each element of the
  array. Generally, things that would be done with loops and iteration
  in other languages are just done by gluing filters together in jq.

  It's important to remember that every filter has an input and an
  output. Even literals like "hello" or 42 are filters - they take an
  input but always produce the same literal as output. Operations that
  combine two filters, like addition, generally feed the same input to
  both and combine the results. So, you can implement an averaging
  filter as `add / length` - feeding the input array both to the `add`
  filter and the `length` filter and dividing the results.

  But that's getting ahead of ourselves. :) Let's start with something
  simpler:

manpage_intro: |
  jq(1) -- Command-line JSON processor
  ====================================

  ## SYNOPSIS

  `jq` [<options>...] <filter> [<files>...]

  `jq` can transform JSON in various ways, by selecting, iterating,
  reducing and otherwise mangling JSON documents. For instance,
  running the command `jq 'map(.price) | add'` will take an array of
  JSON objects as input and return the sum of their "price" fields.

  By default, `jq` reads a stream of JSON objects (whitespace
  separated) from `stdin`. One or more <files> may be specified, in
  which case `jq` will read input from those instead.
  
  The <options> are described in the [INVOKING JQ] section, they
  mostly concern input and output formatting. The <filter> is written
  in the jq language and specifies how to transform the input
  document.

  ## FILTERS

manpage_epilogue: |
  ## BUGS

  Presumably. Report them or discuss them at:

      https://github.com/stedolan/jq/issues

  ## AUTHOR

  Stephen Dolan `<mu@netsoc.tcd.ie>`
  
sections:
  - title: Invoking jq
    body: |
      
      jq filters run on a stream of JSON data. The input to jq is
      parsed as a sequence of whitespace-separated JSON values which
      are passed through the provided filter one at a time. The
      output(s) of the filter are written to standard out, again as a
      sequence of whitespace-separated JSON data.

      You can affect how jq reads and writes its input and output
      using some command-line options:

      * `--slurp`/`-s`:
  
        Instead of running the filter for each JSON object in the
        input, read the entire input stream into a large array and run
        the filter just once.
      
      * `--raw-input`/`-R`:
      
        Don't parse the input as JSON. Instead, each line of text is
        passed to the filter as a string. If combined with `--slurp`,
        then the entire input is passed to the filter as a single long
        string.
  
      * `--null-input`/`-n`:
  
        Don't read any input at all! Instead, the filter is run once
        using `null` as the input. This is useful when using jq as a
        simple calculator or to construct JSON data from scratch.
  
      * `--compact-output` / `-c`:
  
        By default, jq pretty-prints JSON output. Using this option
        will result in more compact output by instead putting each
        JSON object on a single line.
  
      * `--colour-output` / `-C` and `--monochrome-output` / `-M`:
      
        By default, jq outputs colored JSON if writing to a
        terminal. You can force it to produce color even if writing to
        a pipe or a file using `-C`, and disable color with `-M`.
  
      * `--ascii-output` / `-a`:
  
        jq usually outputs non-ASCII Unicode codepoints as UTF-8, even
        if the input specified them as escape sequences (like
        "\u03bc"). Using this option, you can force jq to produce pure
        ASCII output with every non-ASCII character replaced with the
        equivalent escape sequence.

      * `--unbuffered`

        Flush the output after each JSON object is printed (useful if
        you're piping a slow data source into jq and piping jq's
        output elsewhere).

      * `--sort-keys` / `-S`:
        
        Output the fields of each object with the keys in sorted order.

      * `--raw-output` / `-r`:
  
        With this option, if the filter's result is a string then it
        will be written directly to standard output rather than being
        formatted as a JSON string with quotes. This can be useful for
        making jq filters talk to non-JSON-based systems.

      * `-f filename` / `--from-file filename`:

        Read filter from the file rather than from a command line, like
        awk's -f option. You can also use '#' to make comments.

      * `-e` / `--exit-status`:

        Sets the exit status of jq to 10 if the last output values was
        `false`, 11 if the last output value was `null`, or 0 if the
        last output value was valid. Normally jq exits with 0, or 1 if
        there was any usage problem or other error.

      * `--arg name value`:

        This option passes a value to the jq program as a predefined
        variable. If you run jq with `--arg foo bar`, then `$foo` is
        available in the program and has the value `"bar"`.

      * `--argfile name filename`:

        This option passes the first value from the named file as a
        value to the jq program as a predefined variable. If you run jq
        with `--argfile foo bar`, then `$foo` is available in the
        program and has the value resulting from parsing the content of
        the file named `bar`.

  - title: Basic filters
    entries:
      - title: "`.`"
        body: |
          
          The absolute simplest (and least interesting) filter
          is `.`. This is a filter that takes its input and
          produces it unchanged as output.

          Since jq by default pretty-prints all output, this trivial
          program can be a useful way of formatting JSON output from,
          say, `curl`.

        examples:
          - program: '.'
            input: '"Hello, world!"'
            output: ['"Hello, world!"']

      - title: "`.foo`"
        body: |
          
          The simplest *useful* filter is `.foo`. When given a
          JSON object (aka dictionary or hash) as input, it produces
          the value at the key "foo", or null if there's none present.

          If the key contains special characters, you need to surround
          it with double quotes like this: `."foo$"`.
          
        examples:
          - program: '.foo'
            input: '{"foo": 42, "bar": "less interesting data"}'
            output: [42]
          - program: '.foo'
            input: '{"notfoo": true, "alsonotfoo": false}'
            output: ['null']
          - program: '."foo"'
            input: '{"foo": 42}'
            output: [42]

      - title: "`.[foo]`, `.[2]`, `.[10:15]`"
        body: |
          
          You can also look up fields of an object using syntax like
          `.["foo"]` (.foo above is a shorthand version of this). This
          one works for arrays as well, if the key is an
          integer. Arrays are zero-based (like javascript), so `.[2]`
          returns the third element of the array.

          The `.[10:15]` syntax can be used to return a subarray of an
          array or substring of a string. The array returned by
          `.[10:15]` will be of length 5, containing the elements from
          index 10 (inclusive) to index 15 (exclusive). Either index may
          be negative (in which case it counts backwards from the end of
          the array), or omitted (in which case it refers to the start
          or end of the array).

        examples:
          - program: '.[0]'
            input: '[{"name":"JSON", "good":true}, {"name":"XML", "good":false}]'
            output: ['{"name":"JSON", "good":true}']

          - program: '.[2]'
            input: '[{"name":"JSON", "good":true}, {"name":"XML", "good":false}]'
            output: ['null']

          - program: '.[2:4]'
            input: '["a","b","c","d","e"]'
            output: ['["c", "d"]']

          - program: '.[2:4]'
            input: '"abcdefghi"'
            output: ['"cd"']
          
          - program: '.[:3]'
            input: '["a","b","c","d","e"]'
            output: ['["a", "b", "c"]']

          - program: '.[-2:]'
            input: '["a","b","c","d","e"]'
            output: ['["d", "e"]']

      - title: "`.[]`"
        body: |
          
          If you use the `.[foo]` syntax, but omit the index
          entirely, it will return *all* of the elements of an
          array. Running `.[]` with the input `[1,2,3]` will produce the
          numbers as three separate results, rather than as a single
          array.

          You can also use this on an object, and it will return all
          the values of the object.

        examples:
          - program: '.[]'
            input: '[{"name":"JSON", "good":true}, {"name":"XML", "good":false}]'
            output:
              - '{"name":"JSON", "good":true}'
              - '{"name":"XML", "good":false}'

          - program: '.[]'
            input: '[]'
            output: []

          - program: '.[]'
            input: '{"a": 1, "b": 1}'
            output: ['1', '1']

      - title: "`,`"
        body: |
          
          If two filters are separated by a comma, then the
          input will be fed into both and there will be multiple
          outputs: first, all of the outputs produced by the left
          expression, and then all of the outputs produced by the
          right. For instance, filter `.foo, .bar`, produces
          both the "foo" fields and "bar" fields as separate outputs.

        examples:
          - program: '.foo, .bar'
            input: '{"foo": 42, "bar": "something else", "baz": true}'
            output: ['42', '"something else"']

          - program: ".user, .projects[]"
            input: '{"user":"stedolan", "projects": ["jq", "wikiflow"]}'
            output: ['"stedolan"', '"jq"', '"wikiflow"']            
            
          - program: '.[4,2]'
            input: '["a","b","c","d","e"]'
            output: ['"e"', '"c"']
          
      - title: "`|`"
        body: |
          The | operator combines two filters by feeding the output(s) of
          the one on the left into the input of the one on the right. It's
          pretty much the same as the Unix shell's pipe, if you're used to
          that. 

          If the one on the left produces multiple results, the one on
          the right will be run for each of those results. So, the
          expression `.[] | .foo` retrieves the "foo" field of each
          element of the input array.

        examples:
          - program: '.[] | .name'
            input: '[{"name":"JSON", "good":true}, {"name":"XML", "good":false}]'
            output: ['"JSON"', '"XML"']

  - title: Types and Values
    body: |
      
      jq supports the same set of datatypes as JSON - numbers,
      strings, booleans, arrays, objects (which in JSON-speak are
      hashes with only string keys), and "null".

      Booleans, null, strings and numbers are written the same way as
      in javascript. Just like everything else in jq, these simple
      values take an input and produce an output - `42` is a valid jq
      expression that takes an input, ignores it, and returns 42
      instead.

    entries:
      - title: Array construction - `[]`
        body: |
        
          As in JSON, `[]` is used to construct arrays, as in
          `[1,2,3]`. The elements of the arrays can be any jq
          expression. All of the results produced by all of the
          expressions are collected into one big array. You can use it
          to construct an array out of a known quantity of values (as
          in `[.foo, .bar, .baz]`) or to "collect" all the results of a
          filter into an array (as in `[.items[].name]`)
        
          Once you understand the "," operator, you can look at jq's array
          syntax in a different light: the expression `[1,2,3]` is not using a
          built-in syntax for comma-separated arrays, but is instead applying
          the `[]` operator (collect results) to the expression 1,2,3 (which
          produces three different results).

          If you have a filter `X` that produces four results,
          then the expression `[X]` will produce a single result, an
          array of four elements.

        examples:
          - program: "[.user, .projects[]]"
            input: '{"user":"stedolan", "projects": ["jq", "wikiflow"]}'
            output: ['["stedolan", "jq", "wikiflow"]']
      - title: Objects - `{}`
        body: |

          Like JSON, `{}` is for constructing objects (aka
          dictionaries or hashes), as in: `{"a": 42, "b": 17}`.
          
          If the keys are "sensible" (all alphabetic characters), then
          the quotes can be left off. The value can be any expression
          (although you may need to wrap it in parentheses if it's a
          complicated one), which gets applied to the {} expression's
          input (remember, all filters have an input and an
          output).
          
              {foo: .bar}
          
          will produce the JSON object `{"foo": 42}` if given the JSON
          object `{"bar":42, "baz":43}`. You can use this to select
          particular fields of an object: if the input is an object
          with "user", "title", "id", and "content" fields and you
          just want "user" and "title", you can write
          
              {user: .user, title: .title}
          
          Because that's so common, there's a shortcut syntax: `{user, title}`.
          
          If one of the expressions produces multiple results,
          multiple dictionaries will be produced. If the input's
          
              {"user":"stedolan","titles":["JQ Primer", "More JQ"]}
          
          then the expression
          
              {user, title: .titles[]}
          
          will produce two outputs: 
          
              {"user":"stedolan", "title": "JQ Primer"}
              {"user":"stedolan", "title": "More JQ"}
          
          Putting parentheses around the key means it will be evaluated as an
          expression. With the same input as above,
          
              {(.user): .titles}
          
          produces
          
              {"stedolan": ["JQ Primer", "More JQ"]}
              
        examples:
          - program: '{user, title: .titles[]}'
            input: '{"user":"stedolan","titles":["JQ Primer", "More JQ"]}'
            output:
              - '{"user":"stedolan", "title": "JQ Primer"}'
              - '{"user":"stedolan", "title": "More JQ"}'
          - program: '{(.user): .titles}'
            input: '{"user":"stedolan","titles":["JQ Primer", "More JQ"]}'
            output: ['{"stedolan": ["JQ Primer", "More JQ"]}']

  - title: Builtin operators and functions
    body: |

      Some jq operator (for instance, `+`) do different things
      depending on the type of their arguments (arrays, numbers,
      etc.). However, jq never does implicit type conversions. If you
      try to add a string to an object you'll get an error message and
      no result.
      
    entries:
      - title: Addition - `+`
        body: |

          The operator `+` takes two filters, applies them both
          to the same input, and adds the results together. What
          "adding" means depends on the types involved:

          - **Numbers** are added by normal arithmetic.

          - **Arrays** are added by being concatenated into a larger array.

          - **Strings** are added by being joined into a larger string.
          
          - **Objects** are added by merging, that is, inserting all
              the key-value pairs from both objects into a single
              combined object. If both objects contain a value for the
              same key, the object on the right of the `+` wins.
          
          `null` can be added to any value, and returns the other
          value unchanged.

        examples:
          - program: '.a + 1'
            input: '{"a": 7}'
            output: ['8']
          - program: '.a + .b'
            input: '{"a": [1,2], "b": [3,4]}'
            output: ['[1,2,3,4]']
          - program: '.a + null'
            input: '{"a": 1}'
            output: ['1']
          - program: '.a + 1'
            input: '{}'
            output: ['1']
          - program: '{a: 1} + {b: 2} + {c: 3} + {a: 42}'
            input: 'null'
            output: ['{"a": 42, "b": 2, "c": 3}']

      - title: Subtraction - `-`
        body: |

          As well as normal arithmetic subtraction on numbers, the `-`
          operator can be used on arrays to remove all occurences of
          the second array's elements from the first array.

        examples:
          - program: '4 - .a'
            input: '{"a":3}'
            output: ['1']
          - program: . - ["xml", "yaml"]
            input: '["xml", "yaml", "json"]'
            output: ['["json"]']

      - title: Multiplication, division, modulo - `*`, `/`, and `%`
        body: |

          These operators only work on numbers, and do the expected.

          Multiplying a string by a number produces the concatenation of
          that string that many times.

          Dividing a string by another splits the first using the second
          as separators.

        examples:
          - program: '10 / . * 3'
            input: 5
            output: [6]
          - program: '. / ", "'
            input: '"a, b,c,d, e"'
            output: ['["a","b,c,d","e"]']

      - title: `length`
        body: |

          The builtin function `length` gets the length of various
          different types of value:

          - The length of a **string** is the number of Unicode
            codepoints it contains (which will be the same as its
            JSON-encoded length in bytes if it's pure ASCII).

          - The length of an **array** is the number of elements.

          - The length of an **object** is the number of key-value pairs.

          - The length of **null** is zero.

        examples:
          - program: '.[] | length'
            input: '[[1,2], "string", {"a":2}, null]'
            output: [2, 6, 1, 0]

      - title: `keys`
        body: |
          
          The builtin function `keys`, when given an object, returns
          its keys in an array.

          The keys are sorted "alphabetically", by unicode codepoint
          order. This is not an order that makes particular sense in
          any particular language, but you can count on it being the
          same for any two objects with the same set of keys,
          regardless of locale settings.

          When `keys` is given an array, it returns the valid indices
          for that array: the integers from 0 to length-1.

        examples:
          - program: 'keys'
            input: '{"abc": 1, "abcd": 2, "Foo": 3}'
            output: ['["Foo", "abc", "abcd"]']
          - program: 'keys'
            input: '[42,3,35]'
            output: ['[0,1,2]']

      - title: `has`
        body: |
          
          The builtin function `has` returns whether the input object
          has the given key, or the input array has an element at the
          given index.

          `has($key)` has the same effect as checking whether `$key`
          is a member of the array returned by `keys`, although `has`
          will be faster.

        examples:
          - program: 'map(has("foo"))'
            input: '[{"foo": 42}, {}]'
            output: ['[true, false]']
          - program: 'map(has(2))'
            input: '[[0,1], ["a","b","c"]]'
            output: ['[false, true]']

      - title: `to_entries`, `from_entries`, `with_entries`
        body: |
          
          These functions convert between an object and an array of
          key-value pairs. If `to_entries` is passed an object, then
          for each `k: v` entry in the input, the output array
          includes `{"key": k, "value": v}`. 

          `from_entries` does the opposite conversion, and
          `with_entries(foo)` is a shorthand for `to_entries |
          map(foo) | from_entries`, useful for doing some operation to
          all keys and values of an object.

        examples:
          - program: 'to_entries'
            input: '{"a": 1, "b": 2}'
            output: ['[{"key":"a", "value":1}, {"key":"b", "value":2}]']
          - program: 'from_entries'
            input: '[{"key":"a", "value":1}, {"key":"b", "value":2}]'
            output: ['{"a": 1, "b": 2}']
          - program: 'with_entries(.key |= "KEY_" + .)'
            input: '{"a": 1, "b": 2}'
            output: ['{"KEY_a": 1, "KEY_b": 2}']


      - title: `select`
        body: |
          
          The function `select(foo)` produces its input unchanged if
          `foo` returns true for that input, and produces no output
          otherwise.

          It's useful for filtering lists: `[1,2,3] | map(select(. >= 2))`
          will give you `[3]`.

        examples:
          - program: 'map(select(. >= 2))'
            input: '[1,5,3,0,7]'
            output: ['[5,3,7]']

      - title: `empty`
        body: |
          
          `empty` returns no results. None at all. Not even `null`.

          It's useful on occasion. You'll know if you need it :)

        examples:
          - program: '1, empty, 2'
            input: 'null'
            output: [1, 2]
          - program: '[1,2,empty,3]'
            input: 'null'
            output: ['[1,2,3]']

      - title: `map(x)`
        body: |

          For any filter `x`, `map(x)` will run that filter for each
          element of the input array, and produce the outputs a new
          array. `map(.+1)` will increment each element of an array of numbers.

          `map(x)` is equivalent to `[.[] | x]`. In fact, this is how
          it's defined.

        examples:
          - program: 'map(.+1)'
            input: '[1,2,3]'
            output: ['[2,3,4]']

      - title: `add`
        body: |

          The filter `add` takes as input an array, and produces as
          output the elements of the array added together. This might
          mean summed, concatenated or merged depending on the types
          of the elements of the input array - the rules are the same
          as those for the `+` operator (described above).

          If the input is an empty array, `add` returns `null`.

        examples:
          - program: add
            input: '["a","b","c"]'
            output: ['"abc"']
          - program: add
            input: '[1, 2, 3]'
            output: [6]
          - program: add
            input: '[]'
            output: ["null"]

      - title: `range`
        body: |
          
          The `range` function produces a range of numbers. `range(4;10)`
          produces 6 numbers, from 4 (inclusive) to 10 (exclusive). The numbers
          are produced as separate outputs. Use `[range(4;10)]` to get a range as 
          an array.

        examples:
          - program: 'range(2;4)'
            input: 'null'
            output: ['2', '3']
          - program: '[range(2;4)]'
            input: 'null'
            output: ['[2,3]']
            
      - title: `floor`
        body: |

          The `floor` function returns the floor of its numeric input.

        examples:
          - program: 'floor'
            input: '3.14159'
            output: ['3']
            
      - title: `sqrt`
        body: |

          The `sqrt` function returns the square root of its numeric input.

        examples:
          - program: 'sqrt'
            input: '9'
            output: ['3']
            
      - title: `tonumber`
        body: |

          The `tonumber` function parses its input as a number. It
          will convert correctly-formatted strings to their numeric
          equivalent, leave numbers alone, and give an error on all other input.

        examples:
          - program: '.[] | tonumber'
            input: '[1, "1"]'
            output: [1, 1]

      - title: `tostring`
        body: |

          The `tostring` function prints its input as a
          string. Strings are left unchanged, and all other values are
          JSON-encoded.

        examples:
          - program: '.[] | tostring'
            input: '[1, "1", [1]]'
            output: ['"1"', '"1"', '"[1]"']

      - title: `type`
        body: |
          
          The `type` function returns the type of its argument as a
          string, which is one of null, boolean, number, string, array
          or object.

        examples:
          - program: 'map(type)'
            input: '[0, false, [], {}, null, "hello"]'
            output: ['["number", "boolean", "array", "object", "null", "string"]']

      - title: `sort, sort_by`
        body: |
          
          The `sort` functions sorts its input, which must be an
          array. Values are sorted in the following order:

          * `null`
          * `false`
          * `true`
          * numbers
          * strings, in alphabetical order (by unicode codepoint value)
          * arrays, in lexical order
          * objects

          The ordering for objects is a little complex: first they're
          compared by comparing their sets of keys (as arrays in
          sorted order), and if their keys are equal then the values
          are compared key by key.

          `sort_by` may be used to sort by a particular field of an
          object, or by applying any jq filter. `sort_by(foo)`
          compares two elements by comparing the result of `foo` on
          each element.

        examples:
          - program: 'sort'
            input: '[8,3,null,6]'
            output: ['[null,3,6,8]']
          - program: 'sort_by(.foo)'
            input: '[{"foo":4, "bar":10}, {"foo":3, "bar":100}, {"foo":2, "bar":1}]'
            output: ['[{"foo":2, "bar":1}, {"foo":3, "bar":100}, {"foo":4, "bar":10}]']

      - title: `group_by`
        body: |
          
          `group_by(.foo)` takes as input an array, groups the
          elements having the same `.foo` field into separate arrays,
          and produces all of these arrays as elements of a larger
          array, sorted by the value of the `.foo` field.

          Any jq expression, not just a field access, may be used in
          place of `.foo`. The sorting order is the same as described
          in the `sort` function above.

        examples:
          - program: 'group_by(.foo)'
            input: '[{"foo":1, "bar":10}, {"foo":3, "bar":100}, {"foo":1, "bar":1}]'
            output: ['[[{"foo":1, "bar":10}, {"foo":1, "bar":1}], [{"foo":3, "bar":100}]]']

      - title: `min`, `max`, `min_by`, `max_by`
        body: |
          
          Find the minimum or maximum element of the input array. The
          `_by` versions allow you to specify a particular field or
          property to examine, e.g. `min_by(.foo)` finds the object
          with the smallest `foo` field.

        examples:
          - program: 'min'
            input: '[5,4,2,7]'
            output: ['2']
          - program: 'max_by(.foo)'
            input: '[{"foo":1, "bar":14}, {"foo":2, "bar":3}]'
            output: ['{"foo":2, "bar":3}']

      - title: `unique`
        body: |
          
          The `unique` function takes as input an array and produces
          an array of the same elements, in sorted order, with
          duplicates removed.

        examples:
          - program: 'unique'
            input: '[1,2,5,3,5,3,1,3]'
            output: ['[1,2,3,5]']

      - title: `reverse`
        body: |
          
          This function reverses an array.

        examples:
          - program: 'reverse'
            input: '[1,2,3,4]'
            output: ['[4,3,2,1]']

      - title: `contains`
        body: |

          The filter `contains(b)` will produce true if b is
          completely contained within the input. A string B is
          contained in a string A if B is a substring of A. An array B
          is contained in an array A is all elements in B are
          contained in any element in A. An object B is contained in
          object A if all of the values in B are contained in the
          value in A with the same key. All other types are assumed to
          be contained in each other if they are equal.

        examples:
          - program: 'contains("bar")'
            input: '"foobar"'
            output: ['true']
          - program: 'contains(["baz", "bar"])'
            input: '["foobar", "foobaz", "blarp"]'
            output: ['true']
          - program: 'contains(["bazzzzz", "bar"])'
            input: '["foobar", "foobaz", "blarp"]'
            output: ['false']
          - program: 'contains({foo: 12, bar: [{barp: 12}]})'
            input: '{"foo": 12, "bar":[1,2,{"barp":12, "blip":13}]}'
            output: ['true']
          - program: 'contains({foo: 12, bar: [{barp: 15}]})'
            input: '{"foo": 12, "bar":[1,2,{"barp":12, "blip":13}]}'
            output: ['false']

      - title: `startswith`
        body: |

          Outputs true if . starts with the given string argument.

        examples:
          - program: '[.[]|startswith("foo")]'
            input: '["fo", "foo", "barfoo", "foobar", "barfoob"]'
            output: ['[false, true, false, true, false]']

      - title: `endswith`
        body: |

          Outputs true if . ends with the given string argument.

        examples:
          - program: '[.[]|endswith("foo")]'
            input: '["foobar", "barfoo"]'
            output: ['[false, true, true, false, false]']

      - title: `explode`
        body: |

          Converts an input string into an array of the string's
          codepoint numbers.

        examples:
          - program: 'explode'
            input: '"foobar"'
            output: ['[102,111,111,98,97,114]']

      - title: `implode`
        body: |

          The inverse of explode.

        examples:
          - program: 'implode'
            input: '[65, 66, 67]'
            output: ['"ABC"']

      - title: `split`
        body: |

          Splits an input string on the separator argument.

        examples:
          - program: 'split(", ")'
            input: '"a, b,c,d, e"'
            output: ['["a","b,c,d","e"]']

      - title: `recurse`
        body: |
          
          The `recurse` function allows you to search through a
          recursive structure, and extract interesting data from all
          levels. Suppose your input represents a filesystem:

              {"name": "/", "children": [
                {"name": "/bin", "children": [
                  {"name": "/bin/ls", "children": []},
                  {"name": "/bin/sh", "children": []}]},
                {"name": "/home", "children": [
                  {"name": "/home/stephen", "children": [
                    {"name": "/home/stephen/jq", "children": []}]}]}]}
          
          Now suppose you want to extract all of the filenames
          present. You need to retrieve `.name`, `.children[].name`,
          `.children[].children[].name`, and so on. You can do this
          with:

              recurse(.children[]) | .name

        examples:
          - program: 'recurse(.foo[])'
            input: '{"foo":[{"foo": []}, {"foo":[{"foo":[]}]}]}'
            output: 
              - '{"foo":[{"foo":[]},{"foo":[{"foo":[]}]}]}'
              - '{"foo":[]}'
              - '{"foo":[{"foo":[]}]}'
              - '{"foo":[]}'


      - title: "String interpolation - `\(foo)`"
        body: |

          Inside a string, you can put an expression inside parens
          after a backslash. Whatever the expression returns will be
          interpolated into the string.

        examples:
          - program: '"The input was \(.), which is one less than \(.+1)"'
            input: '42'
            output: ['"The input was 42, which is one less than 43"']
          
      - title: "Convert to/from JSON"

          The `tojson` and `fromjson` builtins dump values as JSON texts
          or parse JSON texts into values, respectively.  The tojson
          builtin differs from tostring in that tostring returns strings
          unmodified, while tojson encodes strings as JSON strings.

        examples:
          - program: '[.[]|tostring]'
            input: '[1, "foo", ["foo"]]'
            output: '["1","foo","[\"foo\"]"]'
          - program: '[.[]|tojson]'
            input: '[1, "foo", ["foo"]]'
            output: '["1","\"foo\"","[\"foo\"]"]'
          - program: '[.[]|tojson|fromjson]'
            input: '[1, "foo", ["foo"]]'
            output: '[1,"foo",["foo"]]'

      - title: "Format strings and escaping"
        body: |

          The `@foo` syntax is used to format and escape strings,
          which is useful for building URLs, documents in a language
          like HTML or XML, and so forth. `@foo` can be used as a
          filter on its own, the possible escapings are:

          * `@text`:

            Calls `tostring`, see that function for details.

          * `@json`:

            Serialises the input as JSON.

          * `@html`:

            Applies HTML/XML escaping, by mapping the characters
            `<>&'"` to their entity equivalents `&lt;`, `&gt;`,
            `&amp;`, `&apos;`, `&quot;`.

          * `@uri`:

            Applies percent-encoding, by mapping all reserved URI
            characters to a `%xx` sequence.

          * `@csv`:
            
            The input must be an array, and it is rendered as CSV
            with double quotes for strings, and quotes escaped by
            repetition.

          * `@sh`:
            
            The input is escaped suitable for use in a command-line
            for a POSIX shell. If the input is an array, the output
            will be a series of space-separated strings.
            
          * `@base64`:

            The input is converted to base64 as specified by RFC 4648.

          This syntax can be combined with string interpolation in a
          useful way. You can follow a `@foo` token with a string
          literal. The contents of the string literal will *not* be
          escaped. However, all interpolations made inside that string
          literal will be escaped. For instance,

              @uri "http://www.google.com/search?q=\(.search)"

          will produce the following output for the input
          `{"search":"jq!"}`:

              http://www.google.com/search?q=jq%21

          Note that the slashes, question mark, etc. in the URL are
          not escaped, as they were part of the string literal.

        examples:
          - program: '@html'
            input: '"This works if x < y"'
            output: ['"This works if x &lt; y"']

#          - program: '@html "<span>Anonymous said: \(.)</span>"'
#            input: '"<script>alert(\"lol hax\");</script>"'
#            output: ["<span>Anonymous said: &lt;script&gt;alert(&quot;lol hax&quot;);&lt;/script&gt;</span>"]

          - program: '@sh "echo \(.)"'
            input: "\"O'Hara's Ale\""
            output: ["\"echo 'O'\\\\''Hara'\\\\''s Ale'\""]
        
  - title: Conditionals and Comparisons
    entries:
      - title: `==`, `!=`
        body: |

          The expression 'a == b' will produce 'true' if the result of a and b
          are equal (that is, if they represent equivalent JSON documents) and
          'false' otherwise. In particular, strings are never considered equal
          to numbers. If you're coming from Javascript, jq's == is like
          Javascript's === - considering values equal only when they have the
          same type as well as the same value.
          
          != is "not equal", and 'a != b' returns the opposite value of 'a == b'

        examples:
          - program: '.[] == 1'
            input: '[1, 1.0, "1", "banana"]'
            output: ['true', 'true', 'false', 'false']
      - title: if-then-else
        body: |

          `if A then B else C end` will act the same as `B` if `A`
          produces a value other than false or null, but act the same
          as `C` otherwise.
          
          Checking for false or null is a simpler notion of
          "truthiness" than is found in Javascript or Python, but it
          means that you'll sometimes have to be more explicit about
          the condition you want: you can't test whether, e.g. a
          string is empty using `if .name then A else B end`, you'll
          need something more like `if (.name | length) > 0 then A else
          B end` instead.
          
          If the condition A produces multiple results, it is
          considered "true" if any of those results is not false or
          null. If it produces zero results, it's considered false.
          
          More cases can be added to an if using `elif A then B` syntax.

        examples:
          - program: |-
              if . == 0 then
                "zero"
              elif . == 1 then
                "one"
              else
                "many"
              end
            input: 2
            output: ['"many"']
      
      - title: `>, >=, <=, <`
        body: |
          
          The comparison operators `>`, `>=`, `<=`, `<` return whether
          their left argument is greater than, greater than or equal
          to, less than or equal to or less than their right argument
          (respectively).

          The ordering is the same as that described for `sort`, above.

        examples:
          - program: '. < 5'
            input: 2
            output: ['true']

      - title: and/or/not
        body: |

          jq supports the normal Boolean operators and/or/not. They have the
          same standard of truth as if expressions - false and null are
          considered "false values", and anything else is a "true value".

          If an operand of one of these operators produces multiple
          results, the operator itself will produce a result for each input.

          `not` is in fact a builtin function rather than an operator,
          so it is called as a filter to which things can be piped
          rather than with special syntax, as in `.foo and .bar |
          not`.
          
          These three only produce the values "true" and "false", and
          so are only useful for genuine Boolean operations, rather
          than the common Perl/Python/Ruby idiom of
          "value_that_may_be_null or default". If you want to use this
          form of "or", picking between two values rather than
          evaluating a condition, see the "//" operator below.
          
        examples:
          - program: '42 and "a string"'
            input: 'null'
            output: ['true']
          - program: '(true, false) or false'
            input: 'null'
            output: ['true', 'false']
#          - program: '(true, false) and (true, false)'
#            input: 'null'
#            output: ['true', 'false', 'false', 'false']
          - program: '(true, true) and (true, false)'
            input: 'null'
            output: ['true', 'false', 'true', 'false']
          - program: '[true, false | not]'
            input: 'null'
            output: ['[false, true]']

      - title: Alternative operator - `//`
        body: |

          A filter of the form `a // b` produces the same
          results as `a`, if `a` produces results other than `false`
          and `null`. Otherwise, `a // b` produces the same results as `b`.

          This is useful for providing defaults: `.foo // 1` will
          evaluate to `1` if there's no `.foo` element in the
          input. It's similar to how `or` is sometimes used in Python
          (jq's `or` operator is reserved for strictly Boolean
          operations).

        examples:
          - program: '.foo // 42'
            input: '{"foo": 19}'
            output: [19]
          - program: '.foo // 42'
            input: '{}'
            output: [42]
          
  - title: Advanced features
    body: |
      Variables are an absolute necessity in most programming languages, but
      they're relegated to an "advanced feature" in jq.

      In most languages, variables are the only means of passing around
      data. If you calculate a value, and you want to use it more than once,
      you'll need to store it in a variable. To pass a value to another part
      of the program, you'll need that part of the program to define a
      variable (as a function parameter, object member, or whatever) in
      which to place the data.

      It is also possible to define functions in jq, although this is
      is a feature whose biggest use is defining jq's standard library
      (many jq functions such as `map` and `find` are in fact written
      in jq).

      Finally, jq has a `reduce` operation, which is very powerful but a
      bit tricky. Again, it's mostly used internally, to define some
      useful bits of jq's standard library.

    entries:
      - title: Variables
        body: |
          
          In jq, all filters have an input and an output, so manual
          plumbing is not necessary to pass a value from one part of a program
          to the next. Many expressions, for instance `a + b`, pass their input
          to two distinct subexpressions (here `a` and `b` are both passed the
          same input), so variables aren't usually necessary in order to use a
          value twice.
          
          For instance, calculating the average value of an array of numbers
          requires a few variables in most languages - at least one to hold the
          array, perhaps one for each element or for a loop counter. In jq, it's
          simply `add / length` - the `add` expression is given the array and
          produces its sum, and the `length` expression is given the array and
          produces its length.
          
          So, there's generally a cleaner way to solve most problems in jq than
          defining variables. Still, sometimes they do make things easier, so jq
          lets you define variables using `expression as $variable`. All
          variable names start with `$`. Here's a slightly uglier version of the
          array-averaging example:
          
              length as $array_length | add / $array_length
          
          We'll need a more complicated problem to find a situation where using
          variables actually makes our lives easier.
          
          
          Suppose we have an array of blog posts, with "author" and "title"
          fields, and another object which is used to map author usernames to
          real names. Our input looks like:
          
              {"posts": [{"title": "Frist psot", "author": "anon"},
                         {"title": "A well-written article", "author": "person1"}],
               "realnames": {"anon": "Anonymous Coward",
                             "person1": "Person McPherson"}}
          
          We want to produce the posts with the author field containing a real
          name, as in:
          
              {"title": "Frist psot", "author": "Anonymous Coward"}
              {"title": "A well-written article", "author": "Person McPherson"}
          
          We use a variable, $names, to store the realnames object, so that we
          can refer to it later when looking up author usernames:
          
              .realnames as $names | .posts[] | {title, author: $names[.author]}
          
          The expression `exp as $x | ...` means: for each value of expression
          `exp`, run the rest of the pipeline with the entire original input, and
          with `$x` set to that value.  Thus `as` functions as something of a 
          foreach loop.

          Variables are scoped over the rest of the expression that defines
          them, so 
          
              .realnames as $names | (.posts[] | {title, author: $names[.author]})
          
          will work, but 
          
              (.realnames as $names | .posts[]) | {title, author: $names[.author]}
          
          won't.

        examples:
          - program: '.bar as $x | .foo | . + $x'
            input: '{"foo":10, "bar":200}'
            output: ['210']

      - title: 'Defining Functions'
        body: |
          
          You can give a filter a name using "def" syntax:

              def increment: . + 1;
          
          From then on, `increment` is usable as a filter just like a
          builtin function (in fact, this is how some of the builtins
          are defined). A function may take arguments:

              def map(f): [.[] | f];

          Arguments are passed as filters, not as values. The
          same argument may be referenced multiple times with
          different inputs (here `f` is run for each element of the
          input array). Arguments to a function work more like
          callbacks than like value arguments.

          If you want the value-argument behaviour for defining simple
          functions, you can just use a variable:

              def addvalue(f): f as $value | map(. + $value);

          With that definition, `addvalue(.foo)` will add the current
          input's `.foo` field to each element of the array.

        examples:
          - program: 'def addvalue(f): . + [f]; map(addvalue(.[0]))'
            input: '[[1,2],[10,20]]'
            output: ['[[1,2,1], [10,20,10]]']
          - program: 'def addvalue(f): f as $x | map(. + $x); addvalue(.[0])'
            input: '[[1,2],[10,20]]'
            output: ['[[1,2,1,2], [10,20,1,2]]']

      - title: Reduce
        body: |
        
          The `reduce` syntax in jq allows you to combine all of the
          results of an expression by accumulating them into a single
          answer. As an example, we'll pass `[3,2,1]` to this expression:

              reduce .[] as $item (0; . + $item)
          
          For each result that `.[]` produces, `. + $item` is run to
          accumulate a running total, starting from 0. In this
          example, `.[]` produces the results 3, 2, and 1, so the
          effect is similar to running something like this:

              0 | (3 as $item | . + $item) | 
                  (2 as $item | . + $item) |
                  (1 as $item | . + $item)

        examples:
          - program: 'reduce .[] as $item (0; . + $item)'
            input: '[10,2,5,3]'
            output: ['20']
      

  - title: Assignment
    body: |

      Assignment works a little differently in jq than in most
      programming languages. jq doesn't distinguish between references
      to and copies of something - two objects or arrays are either
      equal or not equal, without any further notion of being "the
      same object" or "not the same object".

      If an object has two fields which are arrays, `.foo` and `.bar`,
      and you append something to `.foo`, then `.bar` will not get
      bigger. Even if you've just set `.bar = .foo`. If you're used to
      programming in languages like Python, Java, Ruby, Javascript,
      etc. then you can think of it as though jq does a full deep copy
      of every object before it does the assignment (for performance,
      it doesn't actually do that, but that's the general idea).

    entries:
      - title: "`=`"
        body: |
          
          The filter `.foo = 1` will take as input an object
          and produce as output an object with the "foo" field set to
          1. There is no notion of "modifying" or "changing" something
          in jq - all jq values are immutable. For instance,
          
           .foo = .bar | .foo.baz = 1
          
          will not have the side-effect of setting .bar.baz to be set
          to 1, as the similar-looking program in Javascript, Python,
          Ruby or other languages would. Unlike these languages (but
          like Haskell and some other functional languages), there is
          no notion of two arrays or objects being "the same array" or
          "the same object". They can be equal, or not equal, but if
          we change one of them in no circumstances will the other
          change behind our backs.
          
          This means that it's impossible to build circular values in
          jq (such as an array whose first element is itself). This is
          quite intentional, and ensures that anything a jq program
          can produce can be represented in JSON.

      - title: "`|=`"
        body: |
          As well as the assignment operator '=', jq provides the "update"
          operator '|=', which takes a filter on the right-hand side and
          works out the new value for the property being assigned to by running
          the old value through this expression. For instance, .foo |= .+1 will
          build an object with the "foo" field set to the input's "foo" plus 1.
          
          This example should show the difference between '=' and '|=':
          
          Provide input '{"a": {"b": 10}, "b": 20}' to the programs:
          
          .a = .b
          .a |= .b
          
          The former will set the "a" field of the input to the "b" field of the
          input, and produce the output {"a": 20}. The latter will set the "a"
          field of the input to the "a" field's "b" field, producing {"a": 10}.

      - title: "`+=`, `-=`, `*=`, `/=`, `%=`, `//=`"
        body: |

          jq has a few operators of the form `a op= b`, which are all
          equivalent to `a |= . op b`. So, `+= 1` can be used to increment values.

        examples:
          - program: .foo += 1
            input: '{"foo": 42}'
            output: ['{"foo": 43}']
        
      - title: Complex assignments
        body: |
          Lots more things are allowed on the left-hand side of a jq assignment
          than in most langauges. We've already seen simple field accesses on
          the left hand side, and it's no surprise that array accesses work just
          as well:
          
              .posts[0].title = "JQ Manual"
          
          What may come as a surprise is that the expression on the left may
          produce multiple results, referring to different points in the input
          document:
          
              .posts[].comments |= . + ["this is great"]
          
          That example appends the string "this is great" to the "comments"
          array of each post in the input (where the input is an object with a
          field "posts" which is an array of posts).
          
          When jq encounters an assignment like 'a = b', it records the "path"
          taken to select a part of the input document while executing a. This
          path is then used to find which part of the input to change while
          executing the assignment. Any filter may be used on the
          left-hand side of an equals - whichever paths it selects from the
          input will be where the assignment is performed.
          
          This is a very powerful operation. Suppose we wanted to add a comment
          to blog posts, using the same "blog" input above. This time, we only
          want to comment on the posts written by "stedolan". We can find those
          posts using the "select" function described earlier:
          
              .posts[] | select(.author == "stedolan")
          
          The paths provided by this operation point to each of the posts that
          "stedolan" wrote, and we can comment on each of them in the same way
          that we did before:
          
              (.posts[] | select(.author == "stedolan") | .comments) |=
                  . + ["terrible."]